This invention relates to systems for converting linear video data into binary representation and in particular to a technique and means for processing binary patterns so as to improve pattern measurements.
There exist many schemes for converting linear video type data into simpler binary patterns. These are easier to manipulate and require less data storage. For instance, consider a portion of a video raster, P, which is quantized spatially into m .times. m picture elements (PIXELS), P.sub.ij ; i, j.epsilon.{0,1,2 . . . m}. The video level, or intensity of each PIXEL is represented by a quantized digit or gray level where P.sub.ij .epsilon.{0, g max}. For the sake of illustration, it will be assumed here that m=32 and g max=255. For this particular instance, 8.times.32.times.32=8192 bits of memory would be required to store this original image. On the other hand, if an algorithm is used to generate a binary pattern, the gray level information is compressed into a single bit. This binary pattern can be stored in a smaller memory, is easier to manipulate and requires less bandwidth for transmission. Needless to say, other reasons exist for converting linear video into a binary representation, in particular, separation of targets from background. In such schemes PIXELS which are determined by some algorithm to be representative of targets are replaced with a "1," the others are set to zero. Another scheme is to subject the gray level values of the individual PIXELS to a threshold test. PIXELS with gray level values greater than the threshold, g.sub.th, are replaced by a "1" otherwise by a "0."
The binary pattern is useful for pattern measurements since it is often more representative of the true pattern geometry than the linear video which can be influenced by glints and temperature variations. Characteristics of the pattern are often expressed in terms of measured parameters. Measurements that are of special interest are the centroid (x,y) and the spatial variances (.sigma.x.sup.2, .sigma.y.sup.2, .sigma.xy): ##EQU1## By way of illustration, consider a binary pattern consisting of a single line of "1" PIXELS, P.sub.ki, for i=0 through n.
______________________________________ ##STR1## SIMPLE BINARY PATTERN ______________________________________
The x centroid coordinate is given by equations one and six as: ##EQU2## Note the strong dependence on the length of the line of ones. Now, if the original linear pattern is noisy, the resulting binary pattern may not be a good representation. This is particularly true if a thresholding technique is used to form the modified pattern. Cells whose gray level value is near the threshold will have their gray levels altered by the noise in a manner such that they can appear or disappear from frame to frame, even though the object does not change. PIXELS near the edge of the pattern are particularly sensitive. In such cases the deletion or addition of a simple binary cell will cause the centroid to shift by 1/2 element. For many applications, such errors are intolerable. The present invention comprehends a procedure and an apparatus for implementing this procedure that generates a multilevel or augmented pattern which is less sensitive to the erroneous deletion and/or addition of cells. The method identifies and classifies cells according to their location with respect to the edge of the pattern. All cells composing the pattern are then assigned a value between 0 and 31 depending upon their type and the cell type of the four closest adjoining cells. What results is a pattern of PIXELS with values assigned on the basis of relative location. These values may be interpreted, in a loose sense, as probabilities that a given PIXEL is part of a pattern. Since the edge cells which are more likely to be misassigned, are given less weight, the pattern measurements tend to remain more stable.